<script type="text/javascript">
<!--
function popup(url)
{
 var width  = 300;
 var height = 200;
 var left   = (screen.width  - width)/2;
 var top    = (screen.height - height)/2;
 var params = 'width='+width+', height='+height;
 params += ', top='+top+', left='+left;
 params += ', directories=no';
 params += ', location=no';
 params += ', menubar=no';
 params += ', resizable=no';
 params += ', scrollbars=no';
 params += ', status=no';
 params += ', toolbar=no';
 newwin=window.open(url,'windowname5', params);
 if (window.focus) {newwin.focus()}
 return false;
}
// -->
</script>

<a href="javascript: void(0)"
   onclick="popup('popup.html')">Centered popup window</a>